I take Freud seriously. Sex is charged with primal, pre-verbal feelings of belonging and connectedness. When those feelings get harmed, the individual suffers greatly. Construction, maintenance, and growth of a highly functional and happy human being take hard emotional work, and lots of learning about boundaries and respect for the equal reality of others' feelings.
As to open relationships: If you like them and find them ideal and a good framework for rearing your kids, go for it. Anything goes between adults. I personally can't imagine not feeling deeply wounded by partnerial infidelity. I also find more to gain than to lose in monogamy, both as a lover and a parent. But that's just me (and a couple billion others).